1. Field of the Invention
The present invention pertains to a redundant storage virtualization controller subsystem and computer system using the same, particularly pertaining to redundant storage virtualization controller subsystem having a storage virtualization controller using SAS device-side IO device interconnect for connecting between a host system and a direct access storage device and a computer system using the same.
2. Description of the Prior Art
Storage virtualization is a technology that has been used to virtualize physical storage by combining sections of physical storage devices (PSDs) into logical storage entities, herein referred to as logical media units (LMUs), that are made accessible to a host system. This technology has been used primarily in redundant arrays of independent disks (RAID) storage virtualization, which combines smaller physical storage devices into larger, fault tolerant, higher performance logical media units via RAID technology.
A Storage virtualization Controller, abbreviated SVC, is a device the primary purpose of which is to map combinations of sections of physical storage media to logical media units visible to a host system. IO requests received from the host system are parsed and interpreted and associated operations and data are translated into physical storage device IO requests. This process may be indirect with operations cached, delayed (e.g., write-back), anticipated (read-ahead), grouped, etc., to improve performance and other operational characteristics so that a host IO request may not necessarily result directly in physical storage device IO requests in a one-to-one fashion.
An External (sometimes referred to as “Stand-alone”) Storage Virtualization Controller is a Storage Virtualization Controller that connects to the host system via an IO interface and that is capable of supporting connection to devices that reside external to the host system and, in general, operates independently of the host.
One example of an external Storage Virtualization Controller is an external, or stand-alone, direct-access RAID controller. A RAID controller combines sections on one or multiple physical direct access storage devices (DASDs), the combination of which is determined by the nature of a particular RAID level, to form logical media units that are contiguously addressable by a host system to which the logical media unit is made available. A single RAID controller will typically support multiple RAID levels so that different logical media units may consist of sections of DASDs combined in different ways by virtue of the different RAID levels that characterize the different units.
Another example of an external Storage Virtualization Controller is a JBOD emulation controller. A JBOD, short for “Just a Bunch of Drives”, is a set of physical DASDs that connect directly to a host system via one or more a multiple-device IO device interconnect channels. DASDs that implement point-to-point IO device interconnects to connect to the host system (e.g., Parallel ATA HDDs, Serial ATA HDDs, etc.) cannot be directly combined to form a “JBOD” system as defined above for they do not allow the connection of multiple devices directly to the IO device channel. An intelligent “JBOD emulation” device can be used to emulate multiple multiple-device IO device interconnect DASDs by mapping IO requests to physical DASDs that connect to the JBOD emulation device individually via the point-to-point IO-device interconnection channels.
Another example of an external Storage Virtualization Controller is a controller for an external tape backup subsystem.
The primary function of a storage virtualization controller, abbreviated as SVC, is to manage, combine, and manipulate physical storage devices in such a way as to present them as a set of logical media units to the host. Each LMU is presented to the host as if it were a directly-connected physical storage device (PSD) of which the LMU is supposed to be the logical equivalent. In order to accomplish this, IO requests sent out by the host to be processed by the SVC that will normally generate certain behavior in an equivalent PSD also generate logically equivalent behavior on the part of the SVC in relation to the addressed logical media unit. The result is that the host “thinks” it is directly connected to and communicating with a PSD when in actuality the host is connected to a SVC that is simply emulating the behavior of the PSD of which the addressed logical media unit is the logical equivalent.
In order to achieve this behavioral emulation, the SVC maps IO requests received from the host to logically equivalent internal operations. Some of these operations can be completed without the need to directly generate any device-side IO requests to device-side PSDs. Among these are operations that are processed internally only, without ever the need to access the device-side PSDs. The operations that are initiated as a result of such IO requests will herein be termed “internally-emulated operations”.
There are operations that cannot be performed simply through internal emulation and yet may not directly result in device-side PSD accesses. Examples of such include cached operations, such as data read operations in which valid data corresponding to the media section addressed by the IO request currently happens to reside entirely in the SVC's data cache, or data write operations when the SVC's cache is operating in write-back mode so that data is written into the cache only at first, to be committed to the appropriate PSDs at a future time. Such operations will be referred to as “asynchronous device operations” (meaning that any actual IO requests to device-side PSDs that must transpire in order for the requested operation to achieve its intended goal are indirectly performed either prior or subsequent to the operation rather than directly in response to the operation).
Yet another class of operations consists of those that directly generate device-side IO requests to PSDs in order to complete. Such operations will be referred to as “synchronous device operations”.
Some host-side IO requests may map an operation that may consist of multiple sub-operations of different classes, including internally-emulated, asynchronous device and/or synchronous device operations. An example of a host-side IO request that maps to a combination of asynchronous and synchronous device operations is a data read request that addresses a section of media in the logical media unit part of whose corresponding data currently resides in cache and part of whose data does not reside in cache and therefore must be read from the PSDs. The sub-operation that takes data from the cache is an asynchronous one because the sub-operation does not directly require device-side PSD accesses to complete, however, does indirectly rely on results of previously-executed device-side PSD accesses. The sub-operation that reads data from the PSDs is a synchronous one, for it requires direct and immediate device-side PSD accesses in order to complete.
A pair of SVCs can be configured into a pair of redundant SVCs, of which the primary motivation is to allow continued, uninterrupted access to data by a host (or more than one host) even in the event of a malfunction or failure of a single SVC. This is accomplished by incorporating functionality into the SVCs that allow one controller to take over for the other in the event that the other becomes handicapped or completely incapacitated. A storage virtualization subsystem has such configuration hereinafter is referred to a redundant storage virtualization subsystem.
On the device side, this requires that both controllers are able to access all of the physical storage devices (PSDs) that are being managed by the SVCs, no matter which SVC any given PSD may initially be assigned to be managed by. On the host side, this requires that each SVC have the ability to present and make available to the host all accessible resources, including those that were originally assigned to be managed by the alternate SVC, in the event that its mate does not initially come on line or goes off line at some point (e.g., due to a malfunction/failure, maintenance operation, etc.).
A typical device-side implementation of this would be one in which device-side IO device interconnects are of the multiple-initiator, multiple-device kind (such as Fibre, Parallel SCSI), and all device-side IO device interconnects are connected to both SVCs such that either SVC can access any PSD connected on a device-side IO device interconnect. When both SVCs are on-line and operational, each PSD would be managed by one or the other SVC, typically determined by user setting or configuration. As an example, all member PSDs of a logical media unit (LMU) that consists of a RAID combination of PSDs would be managed by the particular SVC to which the logical media unit itself is assigned.
A typical host-side implementation would consist of multiple-device IO device interconnects to which the host (s) and both SVCs are connected and, for each interconnect, each SVC would present its own unique set of device IDs, to which LMUs are mapped. If a particular SVC does not come on line or goes off line, the on-line SVC presents both sets of device IDs on the host-side interconnect, its own set together with the set normally assigned to its mate, and maps LMUs to these IDs in the identical way they are mapped when both SVCs are on-line and fully operational. In this kind of implementation, no special functionality on the part of the host that switches over from one device/path to another is required to maintain access to all logical media units in the event that an SVC is not on-line. This kind of implementation is commonly referred to as “transparent” redundancy.
Redundant SVC configurations are typically divided into two categories. The first is “active-standby” in which one SVC is presenting, managing, and processing all IO requests for all logical media units in the storage virtualization subsystem (abbreviated SVS) while the other SVC simply stands by ready to take over in the event that the active SVC becomes handicapped or incapacitated. The second is “active-active” in which both SVCs are presenting, managing, and processing IO requests for the various LMUs that are present in the SVS concurrently. In active-active configurations, both SVCs are always ready to take over for the other in the event that one malfunctions, causing it to become handicapped or incapacitated. Active-active configurations typically provide better levels of performance because the resources of both SVCs (e.g., CPU time, internal bus bandwidth, etc.) can be brought to bear in servicing IO requests rather than the resources of only one SVC.
Another essential element of a redundant storage virtualization system is the ability for each SVC to monitor the status of the other. Typically, this would be accomplished by implementing an inter-controller communications channel (abbreviated ICC) between the two SVCs over which they can exchange the operating status. This communications channel may be dedicated, the sole function of which is to exchange parameters and data relating to the operation of the redundant storage virtualization subsystem, or it can be one or more of the IO device interconnects, host-side or device-side, over which operational parameters and data exchange are multiplexed together with host-SVC or device-SVC IO-request-associated data on these interconnects.
Yet another important element of a redundant storage virtualization system is the ability of one SVC to completely incapacitate the other so that it can completely take over for the other SVC without interference. For example, for the surviving SVC to take on the identity of its mate, it may need to take on the device IDs that the SVC going off line originally presented on the host-side IO device interconnect, which, in turn, requires that the SVC going off line relinquish its control over those IDs.
This “incapacitation” is typically accomplished by the assertion of reset signal lines on the controller being taken off line bringing all externally connected signal lines to a pre-defined state that eliminates the possibility of interference with the surviving SVC. Interconnecting reset lines between the SVCs so that one can reset the other in this event is one common way of achieving this. Another way to accomplish this is to build in the ability of an SVC to detect when itself may be malfunctioning and “kill” itself by asserting its own reset signals (e.g., inclusion of a “watchdog” timer that will assert a reset signal should the program running on the SVC fail to poll it within a predefined interval), bringing all externally connected signal lines to a pre-defined state that eliminates the possibility of interference with the surviving SVC.
Please refer to FIG. 22, where a block diagram of a conventional redundant external storage virtualization computer system is illustrated. Note the interconnection of the host-side IO device interconnects that allows an SVC to take over for its mate by taking over the IO device interconnect IDs that would normally be presented onto the interconnect by its mate and mapping logical media units to these IDs in the same way its mate would. Also, note the interconnection of the device-side IO device interconnects that allow both SVCs access to all PSDs connected to the device-side IO device interconnects. In this example, a typical IO device interconnect that might be used on either host side or device side might be parallel SCSI or Fibre FC-AL, both multiple-initiator, multiple-device IO device interconnects. Therefore, both SVCs operating in target mode (i.e., device mode) are connected to a single interconnect on the host side and allow both SVCs operating in initiator mode, together with multiple devices, to be interconnected on the device side. The configuration shown in FIG. 22 suffers from the drawback that a malfunction of a single PSD, depending on the nature of the malfunction, can potentially bring down an entire device-side IO device interconnect making all other PSDs connected on the same interconnect inaccessible.
FIG. 23 diagrams an improvement on this that effectively avoids the possibility that access to other PSDs connected on the same device-side IO device interconnect might be disrupted due to a malfunction that causes a single device-side interconnect to fail by making use of dual-ported PSDs and adding an additional interconnect to each PSD. In this way, the blockage of a single device-side IO device interconnect, possibly caused by a malfunction of an interconnect controller IC on the PSD, would not result in the inaccessibility of other PSDs connected on the same interconnect because the second interconnect connected to each of the same PSDs can be used to access those PSDs without interference.
The configuration shown in FIG. 23 has the further advantage that IO request load can be distributed between the redundant device-side interconnects thereby effectively doubling the overall bandwidth of the device-side IO device interconnect subsystem as compared to the single-interconnect-per-PSD-set configuration shown in FIG. 22. In this case, the typical device-side IO device interconnect of choice would typically be Fibre FC-AL because of the dual-ported nature of Fibre FC-AL PSDs currently on the market and the elements of the Fibre protocol that allow an initiator, such as an SVC, to determine which interconnect IDs on different interconnects correspond to the same PSD.
While the configuration depicted in FIG. 23 is, indeed, far more robust than that depicted in FIG. 22 in the face of device-side IO device interconnect failure, there is still the possibility that a PSD might malfunction in such a way that it could bring down both IO device interconnects that are connected to its dual-ported port pair. Were this to happen, once again, access to other PSDs connected on the same interconnect pair would be disrupted. In a logical media unit that consists of a standard singly-redundant RAID combination of PSDs (e.g., RAID 5), this could prove disastrous for it can cause multiple PSDs in the combination to go off line causing the entire LMU to go off line.
In the co-pending U.S. patent application Ser. No. 10/707,871, entitled “STORAGE VIRTUALIZATION COMPUTER SYSTEM AND EXTERNAL CONTROLLER THEREFOR,” Ser. No. 10/708,242, entitled “REDUNDANT EXTERNAL STORAGE VIRTUALIZATION COMPUTER SYSTEM,” and Ser. No. 10/709,718, entitled “JBOD SUBSYSTEM AND EXTERNAL EMULATION CONTROLLER THEREOF,” a computer system comprising a SVS implementing SATA interconnects were disclosed in which when using a SATA SVC, a SATA DASD is considered to be the primary DASD.
Therefore, there is a need for a redundant SVS using SAS storage virtualization controller having device-side IO device interconnect port complying with SAS protocol for connecting with DASDs such as SAS DASDs as the primary DASDs thereof. Moreover, there is a need for a SVS that can use a second type of DASD, such as SATA DASD, as a primary DASD rather than the SAS DASDs in addition to the capability of using SAS DASDs as the primary DASDs.